This invention relates to control devices for power tools and, more particularly, to a common speed control selectively applicable to individual tools among a plurality of tools at a work station.
Of particular interest are work stations which use tools wherein the tool bit or tool head is mounted and driven at a location spaced from the primary drive member for the tool.
Numerous tools with rotatable tool heads, for example, are widely used at work stations in industrial craft operations involving the fashioning of jewelry and accessories and in dental offices or laboratories.
A well-known example of the use of such tools is the dental drill which is driven at one location and operated or manipulated by the dentist at another location. Such arrangement is desirable because of the need to manipulate the drill in many directions in a confined area as in the filling of dental cavities. It is noted that, in the case of the dental drill, the primary drive member may be an air turbine.
Another known power tool having a portable electric motor as the primary drive member includes an elongated flexible drive that holds a tool bit remotely from the motor housing in a holding device that permits easy manipulation of the tool bit. The elongated flexible drive separates the tool bit from the motor and facilitates intricate manual operation of the tool bit at desired locations in various different orientations.
One aspect of considerable interest in the use of such power tools is speed control of the tool bit. It is well known that certain types of cutting, grinding and polishing operations are best performed at relatively low rotational speeds, while other tooling operations are best performed at higher rotational speeds.
Various types of speed control devices for electric motors are known, whether the motor be connected directly to a tool bit as in a hand-held drill, or indirectly connected to the tool bit by generation of an air blast for driving a turbine as in the dental drill, or by a flexible rotatable shaft as in a Bowden cable drive for an electric motor. Indeed, such speed controls vary from a rheostat or variable resistor inserted in the power line of a motor to closedloop feedback control as is employed in servomechanisms.
The rheostat is widely employed in speed control devices because of its relatively simple construction, and the facility with which it can be incorporated into existing motor designs. For example, it is common practice to have a trigger-operated rheostat in hand-held electrically-powered tools. Alternatively, the rheostat can be operated by a foot pedal to facilitate manipulation of the tool as is common practice with dental drills.
In many industrial, professional or domestic work stations, in which several tools may be employed, a work project, such as the fashioning of an object from wood or metal often involves several different operations on a particular workpiece. For example, one drill with a large diameter bit and another drill with a small diameter bit may be needed. In addition, a rotatable rasp, a rotatable wire brush, and a rotatable sanding disk may be desired.
In a well-organized work station, several individual power tools, each with its own particular tool head or tool bit are positioned within easy reach of an operator to permit ready selection of one tool or another. However, in most presently known work stations having a plurality of tools, individual speed controls which can be hand or foot operated are provided for each tool.
Tools which require manual speed control by a trigger, for example, are usually more difficult to manipulate than tools having pedal controlled speed because during manual speed control, the weight of the motor is generally borne by the hand which manipulates the tool.
In the case of tools having speed controls operated by a foot pedal, each tool is normally provided with separate foot pedals. A plurality of foot pedals can cause confusion to a tool operator who may find it necessary to glance down below a workbench to ascertain that the proper foot pedal is being activated. A further disadvantage of individual foot pedal controls is that numerous pedal controls are required, even though the tools at a work station are ordinarily operated one at a time.
It is thus desirable to provide a single control for speed actuation of individual selected tools at a work station having a plurality of tools.